Trapdoor spiders are reluctant travellers, but millions of years ago one species appears to have made an epic journey from Africa across the vast Indian Ocean to call Australia home.

Trapdoor spider key points

Key points

The Australian trapdoor spider Moggridgea rainbowi is most closely related to African Moggridgea spiders

The Australian and African species split long after the supercontinent Gondwana separated, and long before humans arrived

This is the first evidence this type of spider may have travelled across the ocean

The Australian trapdoor spider — Moggridgea rainbowi — which is found on Kangaroo Island is famously provincial, rarely moving more than a few metres away from its birthplace for its entire life.

But a study of the spider's genome, published today in PLOS One, shows it split away from its closest relatives in Africa between 2 and 16 million years ago.

There are many Australian species of flora and fauna that share ancestry with African species, says researcher and University of Adelaide PhD candidate Sophie Harrison, from when the two continents were merged in the supercontinent of Gondwana.

"Usually [the Australian and African connection is] because there was a widespread common ancestor and then when the continents drifted apart the distributions remained," Ms Harrison said.

But the genetic analysis of the Australian trapdoor spider and the African Moggridgea spiders indicates they separated long after the supercontinent of Gondwana split up, but before humans arrived in Australia, which rules out human migration as the trapdoors' means of dispersal.

Trapdoor spider territory along the shore of Kangaroo Island in South Australia.

Trapdoor spider territory along the shore of Kangaroo Island in South Australia.

Supplied: Nick Birks

The most likely scenario is that a population of trapdoors drifted across the Indian Ocean on a raft of plant material washed out to sea, then landed on the coastline of Kangaroo Island.

Despite the spider's small size and reluctance to move far from home, Harrison says they are actually quite well suited to dispersal this way.

"Even though they've got these awful dispersal abilities … they're nice and secure in a raft or a chunk of land and they've got a really secure burrow and they've got a really nice fitting trapdoor lid and they've got pretty low metabolic requirements and don't require much resources," she said.

Supporting this hypothesis is the presence of another species of Moggridgea on the volcanic Comoros islands off the south-east coast of Africa, which were formed between 100,000 and 7.7 million years ago.

Ocean journey a first for this spider

Harrison says that if an island is that newly formed, the only way a spider like Moggridgea could have arrived there was by water.

Dispersal over oceans is not uncommon — it has been shown for monkeys, geckos, and even other spiders.

But this is the first time it has been shown for this type of spider, and is particularly surprising given the Moggridgea's reluctance to travel.

Even the genomes of the Kangaroo Island spiders, which were taken from two different groups 80 kilometres apart, showed those groups diverged 1-6 million years ago and have not intermingled.

"They're really prone to being short-range endemic species and having these really restricted distributions, and then suddenly you get this one that's come across the ocean, it's really not what you'd expect," Harrison says.